Cardiovascular disease (CVD) remains our largest cause of mortality and morbidity. Although there are several causes, platelet thrombus formation and fibrinolysis play a central role in the initiation and progression of various facets of these CVDs. Therefore, it is important to understand the cues that cause the breakdown of the regulation of platelet activation. It is my long-term goal to identify and understand the key molecular components involved in proper platelet function and subsequently to develop treatments for patients with acute coronary syndromes. We discovered the platelet surface receptor called "triggering receptor expressed in myeloid cells" (TREM) like transcript-1 or TLT-1 that is found in mice and humans. TLT-1 is abundant, specific to platelets and megakaryocytes, and up-regulated from the alpha-granules upon stimulation with thrombin or LPS to the platelet surface. After platelet activation, a TLT-1 soluble fragment is found in serum. These results suggest that TLT-1 plays a unique role in hemostasis and/or thrombosis. Using single chain antibody fragments (scFv) specific to TLT-1, we are able block thrombin-mediated platelet aggregation and delay clot retraction, suggesting that TLT-1 enhances thrombin-mediated aggregation and is a pro-thrombotic molecule. Clot retraction in platelets is mediated by the CD41/CD61 interaction with its ligand fibrin(ogen). Therefore, the delay in clot retraction implies a possible interaction with CD41/CD61 or its ligand, fibrin(ogen). This project will address the hypothesis that TLT-1 functions as an aggregation receptor secondary to CD41/CD61 whose function is to potentiate the activation of CD41/CD61 and subsequent platelet aggregation. Consequently, TLT-1 plays an early role in the formation and growth of an atherosclerotic plaque. To test this hypothesis, we propose three specific aims that address the mechanisms of TLT-1 involvement in platelet activation and examine TLT-1 function in vivo. Aim 1: To characterize TLT-1 involvement in clot retraction. Aim 2: Identify proteins that interact with the TLT-1 cytoplasmic domain. Aim 3: To elucidate the mechanisms of TLT-1 function and evaluate TLT-1 involvement in the advancement of CVD in an in vivo model. Our discovery and initial characterization of the TLT-1 and its potential role in thrombosis and hemostasis places our laboratory in a unique position to examine this interaction in detail.